1. Field of the Invention
This invention relates to the coating of metal articles and more particularly to coating metal articles, i.e., metal cans, with an aqueous solution of water reducible polyester resins that exhibit superior steam processibility.
2. The Prior Art
In the manufacture of metal containers, a protective synthetic resin coating may be applied to the exterior and interior of the container. The synthetic resins which are employed for coating the metal container are generally heat-curable, resinous materials which are applied in the form of a solution or dispersion in a volatile organic solvent. The wet-coated metal surface is passed through an oven in which hot air is circulated to evaporate the solvent and to cure the coating material to the required hardness. The removal of the organic solvent can create an air pollution problem.
Among the various methods which have been proposed to avoid the use of organic solvents in preparing synthetic resin coatings for metal surfaces is to formulate the coating in a water soluble composition.
Aqueous dispersions of polymers having reactive carboxylic acid groups and heat curable, thermosetting resins have been proposed by the art as metal coatings, e.g. U.S. Pat. No. 2,902,390, U.S. Pat. No. 2,954,358, U.S. Pat. No. 3,053,693, U.S. Pat. No. 3,094,435, U.S. Pat. No. 3,378,477, U.S. Pat. No. 3,403,088, U.S. Pat. No. 3,466,347 and U.S. Pat. No. 3,904,569. These coating materials have not found wide commercial application in the container industry as many of these coatings are not sufficiently inert to the packaged product, or suffer from loss of properties when the metal is fabricated or the can is steam processed or pasteurized.
We have now found novel polyester coating compositions that maintain the good properties of fabrication and steam processibility without loss of coating properties (i.e., adhesion, appearance).